A histamine antagonist is an agent that inhibits action of histamine via histamine receptors. H1 antihistamines are used as treatment for symptoms of allergies such as runny nose. Allergies are caused by an excessive type 1 hypersensitivity response of the body to allergens, such as pollen released by plants. An allergic reaction, which if severe enough can lead to anaphylaxis, results in excessive release of histamines and other mediators by the body. Other uses of H1 antihistamines help with symptoms of local inflammation that results from various conditions, such as insect stings, even if there is no allergic reaction.